cursor: don't access unitialized data when logging
cursor: don't access unitialized data when logging

SpiceCursor::header is only valid when SPICE_CURSOR_FLAGS_NONE is
not set in SpiceCursor::flags, so don't try to log info about
the header before we have tested this flag.

Fantu/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,mathslinux/spice-gtk,guodong/spice-gtk,flexVDI/spice-gtk,SPICE/spice-gtk,guodong/spice-gtk,mathslinux/spice-gtk,fgouget/spice-gtk,Fantu/spice-gtk,flexVDI/spice-gtk,elmarco/spice-gtk,mathslinux/spice-gtk,fgouget/spice-gtk,mathslinux/spice-gtk,SPICE/spice-gtk,SPICE/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,guodong/spice-gtk,guodong/spice-gtk,elmarco/spice-gtk,SPICE/spice-gtk,elmarco/spice-gtk,Fantu/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,fgouget/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,elmarco/spice-gtk,Fantu/spice-gtk